1. Field of the Invention
The present invention relates to a die for drawing molded sheet metal parts, with the die having a bottom part and a top part.
These die parts are movably guided toward each other. One of these die parts has a draw ring. The other part of the die has a sheet metal holder received on a plurality of hydraulically operating cylinders. There is a draw punch, which is adjacent to the sheet metal holder and is supported on at least one hydraulically operating cylinder. The draw punch is communicatively connected with at least one of the operating cylinders serving for the support of the sheet metal holder. The draw punch is acted upon by a hydraulic fluid displaced from this operating cylinder during the drawing process. Thus, the draw punch will move in the direction opposing the movement of the sheet metal holder.
2. The Prior Art
German Patent DE 3,022,844 A1 describes a die which can drive a draw punch in the opposite direction of a draw ring which depends on the downward movement of the draw ring. Thus, it is possible to realize complicated counter draw steps relative to the main drawing process. It is also possible to break up the drawing process into a series of partial drawing steps. The partial drawing steps occur in such a way that part of the drawing depth is achieved through the movement of a draw ring which is lowered by the ram of the drawing press. Another part of the drawing occurs through a movement of the draw punch in the opposite direction.
In this prior art die, as a draw ring is directly actuated by a press ram and is being lowered, there is a pressure medium flowing off from at least one of the pressure cylinders. The pressure cylinder provides support for the sheet metal holder. The pressure medium flows by way of suitable flow paths or conduits into at least one of the pressure cylinders supporting the draw punch, and acts upon the piston guided in the pressure cylinder. This thereby drives the draw punch in the direction opposite to the movement of the draw ring. Another advantage of the known die is that the pressure energy of the pressure medium displaced from the pressure cylinders will support the sheet metal holder as the draw ring is being lowered. The energy resulting from the downward travel of this draw ring is used for driving a draw punch in a direction opposite to the movement of the draw ring.
A problem that keeps recurring during the drawing of molded sheet metal parts with such dies is that the highly undesirable formation of wrinkles and folds may occur in critical drawing areas. These wrinkles can sometimes be eliminated later with difficulty by costly after-treatment of the drawn parts. Sometimes the wrinkles cannot be eliminated. Particularly critical are those drawing areas where the edge of the workpiece is gripped between a sheet metal holder and a draw ring and vibrations occur during the drawing operation. This is the case where wrinkles occur, for example at the corners of sinks having a rectangular cross section.
With these prior art dies, the sheet metal holder always has a sheet metal holder plate, which is received on head plate. The draw ring is normally mounted on a carrier plate. It is known that with such dies that folds potentially developing during the drawing process are avoided by partly increasing within the critical drawing areas the holding force which grips the sheet metal.
It is possible to increase the holding force retaining the sheet metal by placing draw pins underneath the critical areas and to press against sections of the die plates disposed adjacent to such critical areas. It is known also to insert in areas where the formation of folds occurs, strips of paper or sheet metal between the head plate and the sheet metal holder. These strips can be inserted also between the carrier plate and the draw ring.
Pressing against the die plates in the zones disposed adjacent to critical areas does in fact lead to satisfactory results. However, this step is extremely time-consuming and consequently expensive. On the other hand, placing draw pins underneath the critical areas does not always produce the desired results. This is especially true since it is difficult to precisely determine the dimension of such intermediate layers required for eliminating the formations of wrinkles and folds. This applies also to the installation of strips of paper or sheet metal between a sheet metal holder plate and/or a draw ring, on the one hand, and the corresponding parts of the die on the other hand.
Furthermore, this corrective action was found to be extremely time-consuming and costly. This is because the sheet metal holder plate and/or the draw ring have to be removed before such strips of paper or sheet metal can be placed in position. The strip of paper or sheet metal then has to be pushed into the gap between the involved part of the die and the draw ring or sheet metal holder plate. Then the draw ring or the sheet metal holder plate subsequently have to be moved into the correct position again.